lib/modules: change the NewShell factory to take a security.Principal arg.
lib/modules: change the NewShell factory to take a security.Principal arg.

- NewShell now takes a Principal as an argument and allows the
created shell and its subprocesses to share that principal. Taking
this a parameter allows the shell to be used with the runtime's
in-memory Principal when required or with any other.

Change-Id: Ib2dc07fdc4edf1c23afed9b37661eb484f56717e

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23